This invention relates to the general subject of ultraviolet radiation detection and, more specifically, to methods and devices for detection and measurement of exposure to ultraviolet-B radiation, wherein the amount of exposure is measured using either thermoluminescence or optically stimulated luminescence from xcex1-Al2O3:C.
Atmospheric ozone is generally considered to absorb all but approximately 1% of wavelengths below about 320 nm. Recent evidence of ozone depletion in the stratosphere, however, has generated interest in the biological impact on plants and animals resulting from increased exposure to wavelengths below 320 nm. This has created the need for ultraviolet (UV) radiation dosimetry for wavelengths in the UVB region of the electromagnetic spectrum, namely from 320 nm to 280 nm. Specifically, in order to search for possible links between enhanced UVB exposure and potential DNA damage in plants and/or animals the need for a small, portable, integrating UVB dosimeter has arisen. Additionally, the ideal device should also be unaffected by variations in ambient temperatures and by humidity and should be a passivexe2x80x94as opposed to an electronic or activexe2x80x94device, i.e., a device that does not require electrical power while it is operating.
One example of the current state-of-the-art in passive UVB dosimeters is one based on biological indicators such as Bacillus subtilis, pre-Vitamin D and bacteriophage 77. (See, for example, Quintern, L. E., Puskeppeleitm M., Rainer, P., Weber, S., El Naggar, S., Escweiler, U., and Horneck, G. Continuous Dosimetry of the Biologically Harmful UV-Radiation in Antarctica with the Biofilm Technique, in Photochem. Photobiol. B, 22, 59-66 (1994), the disclosure of which is incorporated herein by reference). These sorts of dosimeters are small in size, portable, do not need a power source, and have a linear response to increasing radiation.
Another approach to UV dosimetry measurement involves the use of thermoluminescence (TL). Thermoluminescence is the luminescence emitted from a suitable phosphor when the phosphor is heated following exposure to radiation. The intensity of the TL emitted is a measure of the dose of the absorbed radiation. For UV dosimetry, two approaches are generally used. The first approach is to expose the material directly to UV and then to heat the phosphor immediately after this exposure, yielding a TL signal which is related to the dose of absorbed UV radiation. The alternative approach is to pre-treat the sample by irradiating it with ionizing radiation (such as gamma or beta radiation) which places electronic charge into metastable charge centers (or xe2x80x9cdefectsxe2x80x9d) within the phosphor""s crystal lattice. After the pre-treatment, the sample is exposed to UV radiation, which transfers the electronic charge into defect centers that can be directly stimulated by subsequently heating the sample. During heating, a phototransferred TL (or PTTL) signal is recorded, the intensity of whichxe2x80x94for a given gamma or beta radiation dosexe2x80x94is proportional to the UV exposure. Whether the TL or PTTL approach is used, the sensitivity of the detector to the different wavelengths of UV depends critically upon the material chosen as the phosphor. Previous work by the instant inventors and others (see, for example, Colyott, L. E., Akselrod, M. S. and McKeever, S. W. S., Phototransferred Thermoluminescence xcex1-Al2O3:C_ Radiat. Prot. Dosim 65, 263-266 (1996), the disclosure of which is incorporated herein by reference) has shown that xcex1-Al2O3:C offers many of the favorable properties that one would desire in a UV dosimeter. For example, this material is a sensitive TL radiation detector and it displays a PTTL sensitivity to wavelengths in the UVB range that, along with other desirable properties, make it potentially a versatile base upon which to construct a UVB dosimeter with the desired basic characteristics outlined above.
In addition to its favorable TL and PTTL properties, however, it has been demonstrated in the literature that this material is a sensitive optically stimulated luminescence (OSL) radiation detector (see, for example, Bxc3x8tter-Jensen, L. and McKeever, S. W. S., Optically Stimulated Luminescence Dosimetry Using Natural and Synthetic Materials, Radiat. Prot. Dosim. 65, 273-280 (1996), the disclosure of which is incorporated herein by reference). In the OSL mode of operation, a sample previously irradiated with gamma or beta radiation will luminesce when illuminated with light in the visible range of wavelengths. The luminescence signal is termed OSL and several illumination methods are available, including continuous or steady-state illumination (cw-OSL), pulsed illumination (POSL), and linearly modulated illumination (LM-OSL). Among the many advantages of using an optical stimulation method rather than a thermal stimulation method are that the need for heating the sample is removed. Therefore, the devices that read the luminescence emission require less electrical power to operate; and, most importantly, by stimulating the luminescence emission at low temperature (specifically, ambient temperature) the problem of thermal quenching of the luminescence is avoided. Thermal quenching in xcex1-Al2O3:C is an effect in which the luminescence efficiency decreases as the temperature increases (see, Akselrod, M. S., Whitley, V., Agersnap Larsen, N., and McKeever, S. W. S., Thermal Quenching of Luminescence from Aluminum Oxide, J. Appl. Phys. 84, 3364-3372 (1998), the disclosure of which is incorporated herein by reference). Thus, not only is xcex1-Al2O3:C known to be one of the most sensitive TL phosphors currently available, but it is even more sensitive, in terms of luminescence output per unit absorbed radiation dose, when used as an OSL phosphor.
Thus, it should be clear to those familiar with the UV dosimetry arts that there is, and has been for some time, a need to develop a small, portable, integrating dosimeter capable of sensitively measuring doses of absorbed UVB radiation. Additionally, the resulting dosimeter should be capable of measuring integrated UVB exposures of durations ranging from a few minutes to several days of total exposure with a near-linear response to the total UV exposure. Further, the dosimeter should be capable of measuring UVB exposure in either air or water. Still further, the UV radiation to which the dosimeter has been exposed should be determinable via either a TL technique or an OSL technique. Even further, the dosimeter should exploit the many advantages of using xcex1-Al2O3:C as a UV detecting material. Accordingly, it should be recognized, as was recognized by the present inventors, that there exists, and has existed for some time, a very real need for a device that exhibits the various characteristics described above.
Before proceeding to a detailed description of the present invention, however, it should be noted and remembered that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or preferred embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.
According to a first aspect of the instant invention, there is provided a dosimeter which measures absorbed ultraviolet-B radiation dose for light wavelengths centered at 307 nm and which is based on the phenomenon of the phototransferred luminescence properties (either PTTL, or PT cw-OSL) of xcex1-Al2O3:C. In the preferred embodiment xcex1-Al2O3:C detectors (either in the form of single crystals, thin powder layers on a suitable substrate, polycrystalline chips, or any other form of xcex1-Al2O3:C, including amorphous xcex1-Al2O3:C) are used as the UVB detector. The dosimeter formed therefrom can be used in air or water, and will have a near-linear response with a dynamic range of approximately four decades (over an energy fluence range from about 102 xcexcJ/cm2 to about 106 xcexcJ/cm2). Further, this device exhibits very little temperature dependence in the region of most interest to biological studies (273 K-323 K). The preferred design of the instant dosimeter incorporates a narrow band optical filter, such as an interference filter, to limit the wavelengths of light that reach the dectector. The inherent angular dependence of these sorts of filters is partially offset through the use of diffusers and by the wavelength dependence of the phototransferred luminescence efficiency in the UVB wavelength range and the shift in the transmission wavelength of the filter as a function of incident angle.
According to a second aspect of the instant invention, there is provided a method of preparing detector materials for use in UV detection. By way of general background, operation of the instant device can be explained by reference to the TL properties of xcex1-Al2O3:C. When heated after gamma or beta irradiation, TL is emitted from this material, with a peak of luminescence intensity at temperatures of about 465 K. This effect is due to the thermal release of trapped charge at defects descriptively termed the shallow or xe2x80x9cdosimetry trapsxe2x80x9d. The intensity of the xcx9c465 K TL peak is proportional to the dose of absorbed radiation. More stable traps also exist, which release their trapped electronic charge at temperatures of about 900 K and about 1200 K: the so-called xe2x80x9cdeep trapsxe2x80x9d. If a gamma or beta irradiated sample is heated to just beyond the 465 K TL peak, the dosimetry traps are thermally emptied, but the charge in the deep traps is still present. If, after cooling back to room temperature, the sample is now exposed once again to ultraviolet light, charge from the deep traps is optically stimulated from those traps and a proportion of the charge is retrapped at the empty dosimetry traps. Thus, on second heating a new TL signal at 465 K is now observed, the intensity of which is proportional to the dose of the UV exposure. This TL signal is referred to as the phototransferred thermoluminescence, PTTL. For a given UV exposure, the intensity of the 465 K PTTL signal can be controlled by varying the initial absorbed gamma or beta dose. The efficiency of the UV-induced phototransfer is dependent upon the wavelength of the UV light. This efficiency has been shown to peak in the UVB wavelength range.
As an alternative to thermal stimulation, a luminescence signal can also be induced by optical stimulation. Instead of heating the sample to record the PTTL signal as was described previously, the sample is illuminated with visible light that includes wavelengths that optically stimulate the charge out of the dosimetry traps, thereby creating a luminescence signal which can be measured and correlated with the amount of UV exposure experienced by the detecting material. Experiments have shown that if the illuminating light is centered on wavelengths in the green-blue region of the spectrum, efficient optical stimulation of charge from the dosimetry traps occurs without significant stimulation of charge from the deep traps. Thus, one can measure a phototransferred OSL signal. Since the preferred mode of operation uses continuous visible light illumination, the induced phototransferred luminescence signal is referred as PT cw-OSL. As a result of these considerations the instant design is based upon either UVB-induced PTTL or PT cw-OSL from xcex1-Al2O3:C (although POSL or LM-OSL readout modes could also be employed).
Thus, this aspect of the present invention utilizes the properties of materials such as xcex1-Al2O3:C to create a UVB dosimeter that can be read via PTTL or PT cw-OSLxe2x80x94which will be collectively described as xe2x80x9cphototransferred luminescencexe2x80x9d hereinafter. It should be noted, however, that the OSL readout mode need not be limited to cw-OSL, but rather pulsing (POSL) or linear modulation of the stimulating light (LM-OSL) may also be used, Further, since the underlying physical phenomenon employed is that of measuring the amount of charge that is trapped in the acceptor traps (i.e., how much UV exposure the dosimeter has experienced), any method that provides a measure of this quantity would be acceptable for use with the instant invention.
The foregoing has outlined in broad terms the more important features of the invention so that the detailed description that follows may be more easily understood, and so that the contribution to the art may be better appreciated. The instant invention is not to be limited in its application to the details of the construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. Rather, the invention is capable of other embodiments and of being practiced and carried out in various other ways not specifically enumerated herein. Finally, it should be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting, unless the specification specifically so limits the invention.